I cheated and regret it

[Remorse] It happened two nights ago. I have been with my girlfriend for 5 years straight without one single fight and we are very much in love, not one fight. But this night I was alone with friends and went out with them to a casual bar. One of my friends sister was there and I always thought she was cute but it never even crossed my mind on cheating and much much less with her. Well she started flirting but I did not continue the flirt, at first. We started drinking more and one of my friends got really wasted so we started the 30 min walk home. In my drunken state I wanted to continue the party and told everyone that we should go to club that was near by. Obviously no one cared to go… but her. She immediately responded yes and we went to the club…